The invention relates to porous inorganic membranes. More particularly, the invention relates to a method of making such membranes. Even more particularly, the invention relates to a method of making a porous inorganic membrane coating on a porous ceramic support.
Membranes are porous organic or inorganic films that have separation properties. Inorganic membranes are made of inorganic particles that are partially sintered to form a porous structure. Membranes are classified according to pore size as microfiltration (mean pore size between 0.1 μm and 5 μm) membranes, ultrafiltration (mean pore size between 2 μm and 150 nm) membranes, and nanofiltration (mean pore size between 0.5 μm and 2 nm) membranes. The smaller the pore size of the membrane, the finer the particles that are used to make the membrane.
Inorganic membranes offer several advantages over organic membranes. Inorganic membranes, for example, typically have high chemical and thermal stabilities that allow the membranes to be used in extreme pH and chemical environments. In addition, inorganic membranes can be easily cleaned by applying high temperature treatments such as, for example, firing.
Ultrafiltration membranes are used in the filtration of bacteria, virus, proteins, paint particles, emulsified oil, or protein particles in environmental, food, pharmaceutical, and chemical processing industries. Ultrafiltration membranes are also a necessary underlayer for the deposition of molecular-separation membranes having a mean pore size of less than 1 nm.
Ultrafiltration membranes are typically prepared using a sol-gel process that starts with the preparation of a metal alkoxide solution. The entire process may include hydrolyzing, peptizing, pH-controlling, vaporizing, and sintering to obtain a membrane having a narrow pore size distribution that is controllable from 1-100 nm. Although the sol-gel process is considered one of the best methods for ceramic membrane synthesis, the process is complicated and requires careful control. Moreover, the resulting membrane films have thermal stability problems. Other methods for making ultrafiltration membranes, such as using a cold-plasma to reduce the pore size and repeated chemical vapor deposition and hydroxylation processes, have also been used. The object of such efforts is to simplify the processing steps needed to achieve the desired pore size.
Titanium oxide (TiO2) is commonly used to make such membrane layers. However, since ultrafiltration is usually formed from nanoscale particles, the thermal and hydrothermal stability of these powders become the main challenge. At such small pore and particle sizes, TiO2 membrane material tends to sinter at relatively low temperature, thus reducing the flux through the membrane.
The refractory metal oxide α-alumina (α-Al2O3) is a preferred membrane material. The particle size of α-alumina, however, is usually too large for use in ultrafiltration coatings. Although sufficiently small γ-alumina particles may be formed, this alumina phase is not stable at high temperatures. While γ-alumina can be converted into the α-Al2O3 phase by calcination at temperatures above 1100° C., such high-temperature calcination substantially reduces the porosity of the membrane layer and makes small γ-alumina particles fuse into much larger particles. Alumina ultrafiltration membranes have been formed using a sol-gel based method. These membranes were fired at temperatures of only up to 500° C., however, and thus lack the high-temperature stability and corresponding long-term durability that is required of ultrafiltration and gas-separation membranes.
Current methods are unable to produce membranes that comprise selected materials, such as α-alumina, and are thermally stable. Therefore what is needed is a method of making inorganic membranes that are thermally stable and have a mean pore size and pore size distribution that enable use in microfiltration applications. What is also needed is an inorganic membrane that is thermally stable.